The basic operation and structure of land mobile radio communication systems are known. Such radio communication systems typically comprise a plurality of communication units (vehicle mounted or portable radios in a land mobile system and radio/telephones in a cellular system), a. predetermined number of transceivers, which are located throughout a geographic region and transceive information via communication channels, and a controlling entity. The controlling entity may either be a centralized call processing controller or it may be a network of distributed controllers working together to establish communication paths for the communication units. The communication channels may be time division multiplex (TDM) slots, carrier frequencies, pairs of carrier frequencies or other radio frequency (RF) transmission mediums. A frequency or time portion of one or more of the communication channels may be established for call control purposes such that a communication unit may communicate with the system controller to request and receive system resources.
Multiple site communication systems which comprise a plurality of receivers and transceivers that are distributed throughout a large geographic region are also known. Many multi-site systems use same-frequency simulcast, wherein the same communication channel (or carrier frequency) is used by multiple sites throughout the region to simultaneously relay communications to communication units that are located throughout the multi-site system.
A typical transceiver in a simulcast multi-site communication system comprises an individual circuit that couples the repeater to a central radio system audio collection and distribution point (prime site). Each transceiver receives signals on the same frequency and transports the signals to the prime site where a single signal comparator selects the best signal from all the sites. (Note that a site in the multi-site system may contain a transceiver, i.e. transmitter and receiver, or only a receiver.) The signal selected as the best is distributed from the prime site on links back to the transceiver sites for simultaneous re-transmission. To accurately re-transmit the best signal, dedicated, stable, and time-invariant links have traditionally been used. In such cases, a dedicated (non-switched) link carries messages transmitted between the prime site and each remote site. The link is thus part of a "star" topology network configuration, where the prime site constitutes the center of the star. The links may be analog and/or digital microwave channels. (Note that digital switching networks, for example those provided by public switched telephone setwork (PSTM) operators, have not been used in the past as links because they are not time-invariant.)
With the dedicated, stable, and time invariant links, the site transmitters can re-broadcast the best signal in phase, in time, and on the same frequency such that received signal distortion in overlapping site coverage areas is minimal. The stability of the links ensure that the resulting simulcasted signals remain within acceptable tolerances.
To account for the difference in the physical link transport time delays between the prime site and remote site transmitters, additional adjustable delay circuits are typically added to the links. The adjustable delay circuits compensate for the differences in physical link delay such that the total delay is the same at each transceiver site. This ensures that the signal for transmission arrives at each transceiver site at the exact same time. The adjustable time delay devices added to the transmission distribution links may be at the prime or remote sites.
To accommodate for fluctuations in physical link delays, circuits have been devised to manually or automatically adjust the adjustable time delay circuits. Typically, the channel must be excluded from service while a closed loop test is performed to measure and adjust the delay.
Many users of a simulcast system need immediate and constant access to their system channels. For these users, disabling a channel to conduct a closed loop test is inconvenient at best and potentially catastrophic. Such is certainly the case for Public Safety users and centralized controller systems. In a centralized controller system, if the centralized controller is cut off from the system due to a channel being down, communication units cannot communicate. To avoid this, some systems include duplicate prime site equipment, which involves added logic and switching functions that slows the switch-over process.
Recent technological advances allow time-variant delay links, such as those provided by public digital switching networks, to be used in simulcast systems. To account for the time variations in the links, a maximum delay time is chosen. This allows the user to take advantage of the lower costs in using time varying links but at the cost of system efficiency. A further drawback results because the links are often re-routed in digital networks due to traffic overload or failures. The new route may take a completely different path through different links and switches, even through Earth orbit satellites, and thus have a significantly different delay that exceeds the chosen maximum delay.
Therefore, a need exists for a multi-site simulcast communication system that can efficiently utilize time-invariant or time-variant distribution links, and automatically choose transmitter launch times, without the requirement of a dedicated prime site, on a per call basis.